Wiktionary says it's the genitive, plural of "der" (which I thought was "deren"). I'm not only unsure how to use "derer" but also, what does it mean. Duden doesn't say too much either. Can somebody shed some light, especially with examples?

So it works as a pronoun for something (in this case die Personen), that is used as genitive attribute to something else (in this case die Zahl) AND is itself attributed further or defined in beforehand.

For derer, Wiktionary gives only one example, from an AFP news story. Duden lists many more examples but they all (apparently) are made-up, with no surrounding context that could help to understand usage better. On the plus side, the Duden editors attempt to illuminate when derer is correct and when deren should be used instead.

And Digitales Wörterbuch der deutschen Sprache, which can be so helpful when it shows dozens of examples from actual usage in context, this time falls down completely. So you are left to your own devices. That means firing up ye olde search enginne and doing your own legwork. After looking at a couple of dozen examples, you should be able to get a feel for the word.

Right, the grammar. Well, Duden says that it's genitive plural of der, die, das. That is somewhat confusing. You have to click on a link to find that they do not mean the definite article but der, die, das when they function as demonstratives.

Most people steer well clear of using derer, being unsure of its correct application. Instead of

... derer, die etc.

they will look for a construction such as

... derjenigen, die etc.

... von denen, die etc.

or

... der [ + genitive plural* of the noun that derer was supposed to refer to deictically], die etc.

(This last der, by the way, is the genitive plural of the definite article der, die, das.)

The foregoing does not apply when derer is used as an archaic remnant. Such an example would be

derer von Arnim

given by Duden. No one talks that way anymore except when they

(a) are quoting from 19th-century novels, or

(b) want to affect a highfalutin' tone:

das Geschlecht derer von Arnim

Approximate English equivalent (stick broomstick up posterior and read aloud):

The House of FitzJames (or the House of FitzJames-Stuart) is a noble house of British origin founded by James FitzJames, 1st Duke of Berwick, who was the etc. (Wikipedia).

*identical to nominative plural

Okay, by now you thoroughly hate me, for having filled your head with a mess of stupid detail instead of making the awful German language comprehensible and easy to use. So, forget all of that. There are only three rules. (That's two more than Lt. Jean Rasczak's, but I'm profligate that way.)

(1) Never, ever use derer when speaking or writing.

(2) Bookmark this page and refer back to it when you have the misfortune to encounter derer in a text.

(3) When someone tries to impress you with their refinement by using derer in conversation, LOL @ them.

And there you go. I've done my level best to advise you and other advanced learners of German. If mastery of the German language is a tree, then derer and its uses are the tips of the twiglets of a branch. In my estimation, some 97 percent of Germans do not know at all how to use this demonstrative, or they use it poorly. You would do better to stick to more common alternatives such as derjenigen.

I think the three rules go a bit too far. (Excepting 2, perhaps.) Google News shows about two hundred uses of “derer” in the last 24 hours, and most of them I’d think are perfectly OK. Even the somewhat archaic use for noble families makes an appearance (and I don’t think it’s meant ironic here).
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chirluJun 1 '13 at 8:17